JP2002051550A - Switching power supply and semiconductor device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the efficiency of power supply by a switching power supply without fail by reducing switching loss under light load to reduce power consumption through simple constitution. SOLUTION: A control circuit 15 includes an output load detection circuit 53 that converts feedback signals into feedback voltage signals VFBO indicating the state of a secondary-side load and outputs the signals; an error amplifier 22 that receives the feedback voltage signals VFBO at the inverting terminal thereof, generates error voltage signals VEAO resulting from the difference between the feedback voltage signals and reference voltages applied to the positive-phase terminal thereof, and outputs the signals; a switching signal control circuit 25 that controls switching signal output based on the comparison signals resulting from the comparison of the error voltage signals VEAO with element current detection signals VCL; and a light load detection circuit 40 that causes a switching signal control circuit 25 to stop the output of switching signals to a switching element 14, if the error voltage signal VEAO is lower than the lower limit voltage value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly, to a switching power supply and a semiconductor device for a switching power supply that can reduce power consumption under light load.

[0002]

2. Description of the Related Art First, a conventional switching power supply will be described with reference to the drawings.

FIG. 5 shows a circuit configuration of a conventional switching power supply in which an input side and an output side are electrically insulated. In the switching power supply device shown in FIG. 5, for example, an alternating current input from a commercial power supply to an input terminal is:
It is rectified by a rectifier 101 composed of a diode bridge or the like. Subsequently, the DC voltage is smoothed by the input capacitor 102 and becomes the DC voltage Vin, and the transformer 103 for power conversion is used.
Is input to The transformer 103 includes a first primary winding 10
3a, a second primary winding 103b and a secondary winding 103c, and the generated DC voltage Vin is input to the first primary winding 103a.

The first primary winding 103a of the transformer 103
Is controlled by a switching element 104 composed of a power MOSFET or the like, and the switching operation of the switching element 104 generates an electromotive force in the secondary winding 103c of the transformer 103 by magnetic induction.

The current caused by the electromotive force generated in the secondary winding 103c is equal to the current of the diode 11 connected to the secondary winding 103c.
0 and rectified and smoothed by the output capacitor 111 and supplied to the load 112 as DC power of the output voltage Vo.

The second primary winding 103b of the transformer 103
Also, a DC electromotive force is generated by the first primary winding 103a, and the DC current output from the second primary winding 103b is rectified and smoothed by the auxiliary power supply unit 120 including the diode 121 and the capacitor 122. And an auxiliary power supply voltage Vcc is generated.

[0007] The control circuit 130 driven by the auxiliary power supply voltage Vcc outputs a control signal to the gate of the switching element 104. Here, the auxiliary power supply voltage Vcc is proportional to the output voltage Vo supplied from the secondary winding 103c of the transformer 103 to the load 112, and is also used as a feedback signal for stabilizing the output voltage Vo.

[0008] The control circuit 130 includes the switching element 10
Oscillator 131 which outputs a switching signal to be applied to the oscillator 4
An error amplifier 132 that outputs an error voltage signal VEAO consisting of a difference between the auxiliary power supply voltage Vcc and the reference voltage; and a drain current detection circuit that detects a drain current ID flowing through the switching element 104 and outputs an element current detection signal VCL. 133, a comparator 134 that compares the error voltage signal VEAO with the element current detection signal VCL, and outputs a comparison result.
And a switching signal control circuit 135 for controlling the output of the switching signal based on the comparison signal.

The switching signal control circuit 135 receives a clock signal CLK from the oscillator 131 at a set terminal and an output signal of the comparator 134 at a reset terminal.
The S flip-flop circuit 136 and the oscillator 1
31 receives the maximum duty cycle signal MDC from
A NAND circuit 137 receiving an output signal from the RS flip-flop circuit 136 at another input terminal; a gate driver receiving the output signal of the NAND circuit 137, inverting and amplifying the output signal, and outputting the inverted signal as a control signal to the gate of the switching element 104 138.

Hereinafter, the operation of the conventional switching power supply device configured as described above will be described.

Referring to FIG. 5, first, immediately after the apparatus is started, when an AC current from a commercial power supply is input to the rectifier 101, the input AC current is rectified by the rectifier 101 and the input capacitor 102. The DC voltage Vin is smoothed and converted to a DC voltage Vin, and the converted DC voltage Vin is applied to the first primary winding 103 a of the transformer 103. At this time, the DC voltage Vin is supplied with current through the internal circuit current supply circuit 139 included in the control circuit 130, and the capacitor 122 of the auxiliary power supply unit 120 is charged.

Thereafter, in the auxiliary power supply section 120, when the auxiliary power supply voltage Vcc reaches the starting voltage of the control circuit 130, the control circuit 130 starts operating. Thereby, the control of the switching operation to the switching element 104 is started, and the start / stop circuit 140 stops the internal circuit current supply circuit 139.

The control circuit 130 controls the switching operation of the switching element 104 based on the auxiliary power supply voltage Vcc so that the output voltage Vo to the load 112 is stabilized at a predetermined voltage value. Specifically, the output voltage Vo to the load 112 and the auxiliary power supply voltage Vcc are
3 and a voltage proportional to the turns ratio between the second primary winding 103b and the secondary winding 103c, and an error voltage signal VEAO from the error amplifier 132 and a voltage from the drain current detection circuit 133 to the comparator 134. A comparison is made with the element current detection signal VCL, and when both the signals VEAO and VCL become equal to each other, a high-level output signal is output to the reset terminal of the RS flip-flop circuit 136.

Next, as shown in the timing chart of FIG. 6, when the load supply current decreases to the load 112 and the load supply current Io decreases during a load change, the output voltage Vo slightly increases. In response to this, the auxiliary power supply voltage Vcc of the auxiliary power supply section 120 on the feedback side also increases, and the error amplifier 1
The error voltage signal VEAO from 32 drops.

When the error voltage signal VEAO and the element current detection signal VCL become equal in a state where the error voltage signal VEAO is lowered, such as when no load such as a load change or a standby time and when the load is light, a comparator 134 is used. , A reset signal is output to the reset terminal of the RS flip-flop circuit 136, so that a signal for turning off the switching element 104 is output from the NAND circuit 137 at a timing earlier than during a steady load. As a result, switching element 1
In No. 04, the on-state time during the switching operation is shortened, so that the drain current ID flowing through the switching element 104 decreases.

As described above, the control circuit 130 of the conventional switching power supply device controls the magnitude of the drain current ID flowing through the switching element 104 in accordance with the load supply current Io supplied to the load 112, thereby reducing the lightness. A current mode control method capable of reducing power consumption under load is employed.

[0017]

However, in the conventional switching power supply, the drain current ID flowing through the switching element 104 is reduced during a light load such as during standby, but the drain current ID is completely reduced to zero. Therefore, a certain amount of current flows even when there is no load. Therefore, even when there is no load, the current is lost due to the switching operation of the switching element 104. Therefore, as the load becomes smaller, the ratio of the current loss in the switching element 104 increases. As a result, there is a problem that the efficiency of the power supply is reduced and power saving during standby of the power supply cannot be achieved.

An object of the present invention is to solve the above-mentioned conventional problems. An object of the present invention is to reduce power consumption by reducing switching loss at a light load with a simple configuration, thereby ensuring power supply efficiency in a switching power supply. Is to be able to improve.

[0019]

In order to achieve the above-mentioned object, the present invention provides a switching power supply device which shows a load state on an output side which is detected by an output voltage detection circuit and generated by feedback to a control circuit. The output of the switching signal to the switching element is stopped based on the feedback voltage.

Specifically, a switching power supply according to the present invention includes a transformer, a switching element having an input terminal connected to a primary winding of the transformer, and receiving a first DC voltage via the transformer. Connected to the secondary winding of the transformer,
An output voltage generation circuit that generates and outputs a second DC voltage from the first DC voltage by rectifying and smoothing the output voltage on the secondary side, a control circuit that controls the operation of the switching element, An output voltage detection circuit for detecting and outputting the voltage value of the DC voltage of No. 2, a feedback circuit for insulatedly feeding back the output signal of the output voltage detection circuit to the control circuit as a feedback signal, and an output of the primary winding to the anode. Receiving the output of the switching element on the cathode, a control circuit power supply capacitor for generating a power supply voltage for the control circuit, and provided between the output side of the primary winding and the anode of the control circuit power supply capacitor, A voltage regulator for holding a power supply voltage at a predetermined value; a control circuit for generating and outputting a switching signal to be applied to the switching element; an oscillator for detecting a current flowing through the switching element; A current detection circuit that outputs a current detection signal, an output load detection circuit that converts a feedback signal into a feedback voltage signal that indicates a secondary-side load state, and outputs an error voltage that is a difference between the feedback voltage signal and a reference voltage An error amplifier that generates and outputs a signal, a comparator that compares an element current detection signal and an error voltage signal, and outputs a compared comparison signal, and a switching signal control that controls an output of a switching signal based on the comparison signal When the error voltage signal is smaller than the lower limit voltage value, the output of the switching signal to the switching element is stopped for the switching signal control circuit when the error voltage signal is smaller than the lower limit voltage value. A light load detection circuit that starts outputting a switching signal to the control circuit.

According to the switching power supply of the present invention,
When the current consumed at the time of light load decreases and the second DC voltage which is the output voltage of the device increases, the secondary side output voltage,
That is, the voltage value of the feedback voltage signal reflecting the load state of the second DC voltage increases. As a result, the voltage value of the error voltage signal from the error amplifier that generates the error voltage signal including the difference between the feedback voltage signal and the reference voltage decreases. At this time, when the error voltage signal is smaller than the lower limit voltage value, the light load detection circuit stops outputting the switching signal to the switching element with respect to the switching signal control circuit. Since the power consumption under load can be reduced, the power supply efficiency of the switching power supply can be improved.

In addition, since the voltage regulator is provided between the output side of the primary winding and the anode of the power supply capacitor for the control circuit, the power supply voltage value of the control circuit generated by the power supply capacitor for the control circuit is controlled to a predetermined value. Since the output voltage can be held at the value, the output load detection circuit can reliably convert the feedback signal reflecting the load state of the secondary side from the feedback signal.

In the switching power supply of the present invention,
Preferably, the feedback circuit includes a photocoupler. With this configuration, even when the input side and the output side are electrically insulated by the transformer, the load state on the output side can be reliably fed back to the input side.

In the switching power supply of the present invention,
It is preferable that the light load detection circuit has a detection voltage variable unit that can variably set the value of the lower limit voltage or the upper limit voltage. With this configuration, the load current value during standby can be optimized according to the system in which the present device is to be incorporated, and the number of options for the system to be incorporated can be increased.

In the switching power supply of the present invention,
The switching element and the control circuit are connected to an input terminal and an output terminal of the switching element, and an input terminal on the anode side of the control circuit power supply capacitor in the control circuit and an input terminal of the feedback signal in the output load detection circuit are externally connected. It is preferable that they are integrated on one semiconductor substrate so as to be terminals. With this configuration, since the switching element and the control circuit can be integrated into one chip, the number of components can be significantly reduced, and the size of the switching power supply device can be reduced.

A semiconductor device for a switching power supply according to the present invention includes a transformer, a switching element having an input terminal connected to a primary winding of the transformer, and receiving a first DC voltage via the transformer; Rectifies and smoothes the output voltage on the secondary side of the transformer, and
Output voltage generation circuit for generating and outputting a second DC voltage from the DC voltage of the first DC voltage, a control circuit for controlling the operation of the switching element, and an output voltage detection circuit for detecting and outputting the voltage value of the second DC voltage A feedback circuit that insulates the output signal of the output voltage detection circuit to the control circuit as a feedback signal, receives an output of the primary winding on an anode, receives an output of a switching element on a cathode, and supplies a power supply for the control circuit. A switching power supply device comprising: a power supply capacitor for a control circuit that generates a voltage; and a voltage regulator that is provided between an output side of the primary winding and an anode of the power supply capacitor for the control circuit, and that holds a power supply voltage at a predetermined value. The semiconductor device for a switching power supply includes a switching element and a control circuit, and the control circuit applies a voltage to the switching element. An oscillator for generating and outputting a switching signal, a current detection circuit for detecting a current flowing through the switching element, and outputs as an element current detection signal,
An output load detection circuit that converts the feedback signal into a feedback voltage signal indicating a secondary-side load state and outputs the error voltage signal; an error amplifier that generates and outputs an error voltage signal including a difference between the feedback voltage signal and the reference voltage; A comparator for comparing the element current detection signal and the error voltage signal and outputting a comparison signal compared therewith; a switching signal control circuit for controlling the output of the switching signal based on the comparison signal; When the error voltage signal is smaller than the upper limit voltage value, the output of the switching signal to the switching element is stopped for the switching signal control circuit. And a light load detection circuit to start.

According to the semiconductor device for a switching power supply of the present invention, when the current consumed at the time of light load decreases and the second DC voltage which is the output voltage of the device increases, the secondary output voltage, that is, the second output voltage, The voltage value of the feedback voltage signal reflecting the load state of the DC voltage increases. As a result, the voltage value of the error voltage signal from the error amplifier that generates the error voltage signal including the difference between the feedback voltage signal and the reference voltage decreases.
At this time, when the error voltage signal is smaller than the lower limit voltage value, the light load detection circuit stops outputting the switching signal to the switching element with respect to the switching signal control circuit. Since the power consumption under load can be reduced, the power supply efficiency of the switching power supply can be improved.

In the semiconductor device for a switching power supply according to the present invention, it is preferable that the light load detection circuit has a detection voltage varying means capable of variably setting a lower limit voltage or an upper limit voltage.

In the semiconductor device for a switching power supply according to the present invention, the switching element and the control circuit may include an input terminal and an output terminal of the switching element, and an input terminal and an output load detection circuit on the anode side of the control circuit power supply capacitor in the control circuit. It is preferable that the input terminal of the feedback signal be integrated on one semiconductor substrate so as to be an external connection terminal.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a schematic circuit configuration of a switching power supply according to an embodiment of the present invention. As shown in FIG. 1, the switching power supply according to the present embodiment converts a smoothed first DC voltage applied to a main input terminal 10 from, for example, an AC current from a commercial power supply to a transformer (transformer). 3) While the voltage is being applied to the primary side of the switching element 13, the switching operation by repeating the on / off operation of the switching element 14 causes the output voltage generation circuit 16 provided on the secondary side of the transformer 13 to output the desired output as the second DC voltage The switching power supply device outputs the voltage Vo to the main output terminal 17.

Hereinafter, the switching power supply device will be described in detail.

The transformer 13 has a primary winding 13a and a secondary winding 13c.

A rectifier 11 composed of a diode bridge or the like for rectifying an AC current and an input capacitor 12 for smoothing a rectified signal to generate a DC voltage Vin are connected to the main input terminal 10 in parallel. . After the generated DC voltage Vin is input to the primary winding 13a of the transformer 13, it is input to the drain terminal TD of a switching element 14 composed of, for example, an N-type power MOSFET.
Here, the source terminal Ts of the switching element 14 is connected to the cathode terminal of the input capacitor 12, and a control signal output from a control circuit 15 for controlling the operation of the switching element 14 is input to its gate.

Note that the present switching power supply device has a rectifier 1
1 may not be provided. In that case, the first DC voltage Vin is rectified in advance and then
Should be entered.

An output voltage generating circuit 16 is connected to the secondary winding 13c of the transformer 13. The output voltage generating circuit 16 rectifies a current caused by an electromotive force generated by magnetic induction of the DC voltage Vin applied to the primary winding 13a and switched, and smoothes the rectified signal. And an output capacitor 162.

The main output terminal 17 connected to the output voltage generating circuit 16 has a load 18 connected between its high-level terminal and low-level terminal. The load 18 has a load supply current Io. Flows.

An output voltage detection circuit 51 connected in parallel with the secondary winding 13c of the transformer 13 for detecting an output voltage Vo as a second DC voltage, and a detection signal for the primary side control circuit 15 And a photocoupler as a feedback circuit capable of performing feedback in an insulating state as a feedback signal.

The photocoupler includes a light emitting diode unit 52a connected to the output voltage detection circuit 51 on the secondary side, a feedback signal input terminal TFB and a source terminal TFB of the control circuit 15 on the primary side.
s and the light receiving transistor section 52b connected between the first and second s. Note that the output voltage detection circuit 51 may be configured to include, for example, a series body of a shunt regulator and a resistor.

In this embodiment, the area surrounded by the broken line 20, ie, the switching element 14 and the control circuit 15
A reference voltage terminal TREF and a feedback signal input terminal TFB, which are connected to the drain terminal TD and the source terminal Ts of the switching element 14 and the anode of the control circuit capacitor 19, and to which a reference voltage of the control circuit 15 is applied. Terminals enable input and output with the outside. Here, this 4
An area including the terminals TD, Ts, TREF, and TFB is called an on-substrate formation area 20, which indicates that the on-substrate formation area 20 can be formed on one semiconductor chip. Note that the on-substrate formation region 20 does not necessarily need to be formed into one chip, and may be divided into a plurality of semiconductor substrates. Also in this case, it is preferable that the above-mentioned four terminals TD, Ts, TREF and TFB be housed in one package having external connection terminals.

The control circuit 15 includes an oscillator 21 for generating and outputting a switching signal having an oscillation frequency of, for example, about 100 kHz, which is applied to the switching element 14, and a shared source connected to the reference voltage terminal TREF. The gate is connected to the feedback signal input terminal TREF,
A first P-type MOSFET 531 and a second P-type MO having the other drain connected to the source terminal Ts via a resistor
An output load detection circuit 53 comprising an SFET 532 for converting a feedback signal into a feedback voltage signal VFBO indicating a load state on the secondary side and outputting the feedback voltage signal VFBO; And an error amplifier 22 for generating and outputting an error voltage signal VEAO comprising a difference from a reference voltage applied to the reference voltage. Further, a drain current ID flowing through the switching element 14 is detected, and the detected drain current ID is converted into a voltage.
A drain current detection circuit 23 which outputs the signal as CL, an error voltage signal VEAO and a device current detection signal VCL, and a drain current detection comparator 24 which outputs a compared comparison signal; and a switching signal based on the comparison signal. A switching signal control circuit 25 for controlling the output and a switching element 1 for the switching signal control circuit 25 when the error voltage signal VEAO is smaller than the lower limit voltage value.
And a light load detection circuit 40 that stops outputting the switching signal to the switching signal control circuit 4 and starts outputting the switching signal to the switching signal control circuit 25 when the error voltage signal VEAO is larger than the upper limit voltage value. I have.

Further, the control circuit 15 is connected between the drain terminal TD of the switching element 14 and the reference voltage terminal TREF of the control circuit 15, and has a voltage for maintaining a predetermined reference voltage value with respect to the reference voltage terminal TREF. Regulator 29
And a start / stop circuit 30 connected to the reference voltage terminal TREF and transmitting a high-level enable signal or a low-level disable signal to the switching signal control circuit 25 when the control circuit 15 starts or stops. .

The switching signal control circuit 25 includes an RS flip-flop circuit 26 which receives an output signal of the light load detection circuit 40 at a set terminal S and an output signal of a drain current detection comparator 24 at a reset terminal R, Receives an output signal of the start / stop circuit 30 at an input terminal thereof and a maximum duty cycle signal M from the oscillator 21 at a second input terminal thereof.
A NAND circuit 27 that receives DC and receives an output signal from an RS flip-flop circuit 26 at a third input terminal;
A gate driver 28 comprising an inverter that receives an output signal of the AND circuit 27 and outputs a control signal obtained by inverting and amplifying the received output signal to the gate of the switching element 14.

The light load detection circuit 40 includes a reference voltage source 41
A light load detection comparator 42 which receives the error voltage signal VEAO from the error amplifier 22 at the positive phase input terminal and receives the reference voltage VR from the reference voltage source 41 at the negative phase input terminal, and a light load detection comparator 42 at one input terminal. The output signal of the load detection comparator 42 is received, and the clock signal C from the oscillator 21 is input to the other input terminal.
And an AND circuit 43 receiving the LK. The reference voltage source 41 is configured to be able to change the value of the reference voltage VR in response to the output of the light load detection comparator 42.

The light load detection comparator 42 compares the input error voltage signal VEAO with the reference voltage VR, and when the error voltage signal VEAO is larger than the reference voltage VR,
A high-level signal is output to the AND circuit 43.
Conversely, when the error voltage signal VEAO is smaller than the reference voltage VR, a low-level signal is output to the AND circuit 43, so that the output signal of the RS flip-flop circuit 26 is at a low level. The output of the control signal from 28 can be stopped.

On the output side of the error amplifier 22, there is provided an overcurrent protection circuit 31 composed of a PNP-type bipolar transistor for clamping the maximum value of the error voltage signal VEAO, and the error voltage signal VEAO exceeds the clamp value. In this case, the overcurrent is short-circuited to the source terminal Ts of the switching element 14 so that the switching element 1
4 is protected.

In the switching power supply according to the present embodiment, the voltage values of the DC voltage Vin and the output voltage Vo are not limited. In addition, since the number of components of the switching power supply can be significantly reduced by forming one package and then one chip, the size of the switching power supply can be reduced, and the size and the price can be reduced.

The switching element 14 has an N-type MOS
Although an FET is used, an NPN-type bipolar transistor may be used instead.

Hereinafter, the operation of the switching power supply device configured as described above will be described.

First, the activation of the switching power supply according to this embodiment will be described. When AC power is input to the main input terminal 10, the DC voltage Vin is rectified and smoothed,
The control circuit power supply capacitor 19 is charged via the primary winding 13a of the transformer 13 and the voltage regulator 29. Thereafter, when the reference voltage value of the reference voltage terminal TREF reaches the starting voltage, the control circuit 15 operates and the on / off control of the switching element 14 is started.

Next, the operation of the switching power supply according to the present embodiment when the load changes will be described with reference to FIG.

FIG. 2 shows the operation of the switching power supply of this embodiment in response to a change in the output load state.

As shown in FIG. 2, the rated load (steady load)
At the time, the value of the reference voltage VR of the reference voltage source 41 is set to the lower limit voltage value VR1.

Thereafter, for example, when a load change that causes a light load such that the load supply current Io decreases, the load 18
Becomes excessive, the voltage value of the output voltage Vo slightly increases. This increase in the output voltage Vo is detected by the output voltage detection circuit 51, and this detection signal is sent to the feedback signal input terminal TFB of the control circuit 15 via the light emitting diode section 52a and the light receiving transistor section 52b of the photocoupler.
Is transmitted to Thereby, the light receiving transistor section 52b
As the feedback signal current flowing through the first P-type MOSFET 531 increases, the potentials of the drain and gate of the first P-type MOSFET 531 decrease, and the first and second P-type MOSFETs 531 and 532
Is in a low impedance state. As a result, since the potential of the reference voltage terminal TREF is maintained at a predetermined value via the voltage regulator 29, the second P-type MOSFET 532
The voltage value of the feedback voltage signal VFBO, which is the output signal of, increases. Further, with the rise of the feedback voltage signal VFBO,
Since the voltage value of the error voltage signal VEAO output from the error amplifier 22 decreases, even if the voltage value of the element current detection signal VCL from the drain current detection circuit 23 is low, a high level signal is output from the drain current detection comparator 24. A signal is output. As a result, the timing at which the reset signal is output from the RS flip-flop circuit 26 is advanced, and the voltage value of the element current detection signal VCL gradually decreases.

As described above, the voltage value of the error voltage signal VEAO from the error amplifier 22 is equal to the lower limit voltage value VR for light load detection.
When the load fluctuates until it becomes equal to 1, the switching operation of the switching element 14 does not stop, and the drain current ID flowing through the switching element 14 does not change.
Decreases linearly. As a result, the current consumption of the switching element 14 during a load change is gradually reduced in accordance with the size of the load. This is a feature of the so-called current mode PWM control method.

Next, at a light load when the value of the error voltage signal VEAO is substantially equal to the lower limit voltage value VR1, the light load detection comparator 42 of the light load detection circuit 40
, The gate driver 28 of the switching signal control circuit 25 outputs only the low-level control signal, and the switching operation of the switching element 14 is stopped. At substantially the same time, the output voltage VR of the reference voltage source 41 is changed from the lower limit voltage value VR1 to the upper limit voltage value VR2 in response to the low level output signal of the light load detection comparator 42.

Next, when the load becomes a light load or no load as in a standby state, the switching control for the switching element 14 is stopped, the switching element 14 is turned off, and the drain current ID does not flow through the switching element 14. As a result, power supply to the secondary winding 13c via the primary winding 13a of the transformer 13 is not performed, and power supply to the load 18 is performed only from the output capacitor 162. Vo gradually decreases. When the output voltage Vo drops, the amount of current of the feedback signal detected by the output voltage detection circuit 51 and fed back to the primary side by the photocoupler decreases, so that the feedback voltage signal VFBO output from the output load detection circuit 53 becomes Decreases gradually. At this time, as shown in FIG. 3, the output voltage VR of the reference voltage source 41 is set to the upper limit voltage VR higher than the lower limit voltage VR1.
Since it is set to 2, the switching operation by the switching element 14 is not immediately restarted.

Further, when the output voltage Vo decreases and the error voltage signal VEAO exceeds the upper limit voltage value VR2, the output signal from the light load detection comparator 42 becomes high level again and receives this. Since the AND circuit 43 can output a high-level output signal, the switching operation of the switching element 14 is restarted. At about the same time, the output voltage VR of the reference voltage source 41 is reset from the upper limit voltage value VR2 to the lower limit voltage value VR1 upon receiving the high level output signal of the light load detection comparator 42.

When the switching operation by the switching element 14 is restarted, the power supply to the load 18 becomes excessive because the drain current ID flowing through the switching element 14 is larger than the current value when the light load is detected. Then, the output voltage Vo rises again, and the output load detection circuit 53
Of the feedback voltage signal VFBO from the input terminal is reduced. Therefore, as described above, the error voltage signal VEAO is set to the lower limit voltage value VR1.
When it becomes smaller, the output of the switching signal to the switching element 14 is stopped again.

As described above, when the reference voltage VR output from the reference voltage source 41 detects the light load state, the switching operation is stopped, and the reference voltage VR is further changed from the lower limit voltage value VR1 to the upper limit voltage value VR2. By changing the reference voltage VR so that the switching operation is not immediately started even if the error voltage signal VEAO rises.
Has a hysteresis characteristic. As a result, while the light load or no load is detected, the switching element 14
Is in an intermittent oscillation state in which the switching operation is repeatedly stopped and restarted.

The output voltage Vo drops during the switching stop period in the intermittent oscillation state, and the degree of this drop depends on the load supply current Io. In other words, as the load supply current Io decreases, the output voltage Vo decreases more gradually.

The switching stop period in the intermittent oscillation state becomes longer as the load supply current Io becomes smaller. That is, the switching operation of the switching element 14 decreases as the load becomes lighter.

Further, the circuit configurations of the light load detection circuit 40 and the output load detection circuit 53 shown in FIG. 1 are not limited to these, but may be any circuit configuration having equivalent functions.

(Modification of the Embodiment) Hereinafter, a modification of the embodiment will be described with reference to the drawings.

FIG. 4 shows a schematic circuit configuration of a switching power supply according to a modification of the embodiment. In FIG. 4, the same components as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

As shown in FIG. 4, the switching power supply according to the present modification has a light load detection voltage adjusting terminal TR provided at one end at the end of the on-substrate formation region 20 for light load detection. It has a light load detection voltage adjusting resistor 55 as detection voltage variable means connected to the negative phase input terminal of the comparator 42 and the other end connected to the low level side of the main input terminal 10.

As described above, by providing the light load detection voltage adjusting resistor 55, the lower limit voltage value VR1 and the upper limit voltage value VR2 of the reference voltage VR for light load detection can be appropriately adjusted. It is possible to optimize the load supply current Io when the switching operation of the switching element 14 is stopped or restarted, together with the necessary load during standby. Therefore, the switching element 14 and the control circuit 15
Is integrated into a single chip, the lower limit voltage value VR1 or the upper limit voltage value VR2 of the light load detection circuit 40 can be changed according to the use of the power supply device.

In this modification, the light load detection voltage adjusting resistor 55 is provided outside the on-substrate formation region 20. However, the present invention is not limited to this. It may be incorporated in the upper formation region 20. In this case, the adjustment of the resistance value is performed by trimming technology,
For example, it can be performed by a laser trimming method.

[0069]

According to the switching power supply of the present invention, an error amplifier for outputting an error voltage signal consisting of a difference between a reference voltage and a feedback voltage signal generated by feedback from the output side, and an error voltage signal having a lower limit A light load detection circuit that stops outputting the switching signal to the switching element when the voltage is smaller than the voltage value, so that the loss in the switching element is reduced and the power consumption at the time of light load is reduced. , Power efficiency of the switching power supply semiconductor device can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram showing a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a timing chart illustrating an operation of the switching power supply according to the embodiment of the present invention.

FIG. 3 is a timing chart showing a reference voltage applied to a light load detection comparator in the switching power supply according to the embodiment of the present invention.

FIG. 4 is a schematic circuit diagram showing a switching power supply according to a modification of the embodiment of the present invention.

FIG. 5 is a schematic circuit diagram showing a conventional switching power supply device.

FIG. 6 is a timing chart showing the operation of a conventional switching power supply device.

[Description of Signs] 10 Main input terminal 11 Rectifier 12 Input capacitor 13 Transformer (transformer) 13a Primary winding 13c Secondary winding 14 Switching element 15 Control circuit 16 Output voltage generation circuit 161 First diode 162 Output capacitor 17 Main output terminal 18 Load 19 Power supply capacitor for control circuit 20 Formed area on substrate 21 Oscillator 22 Error amplifier 23 Current detection circuit 24 Drain current detection comparator 25 Switching signal control circuit 26 RS flip-flop circuit 27 NAND circuit 28 Gate driver 29 Voltage Regulator 30 Start / Stop circuit 31 Overcurrent protection circuit 40 Light load detection circuit 41 Reference voltage source 42 Light load detection comparator 43 AND circuit 51 Output voltage detection circuit 52a Light emitting diode section (feedback circuit) 52b Light receiving transistor Data section (feedback circuit) 53 output load detection circuit 531 first P-type MOSFET 532 second P-type MOSFET 55 light load detection voltage adjusting resistor (detection voltage varying means) Ts source terminal TD drain terminal TREF reference voltage terminal TBF Feedback signal input terminal TR Light load detection voltage adjustment terminal

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Seiki Yamaguchi 1-1, Kochi-cho, Takatsuki-shi, Osaka Inside Matsushita Electronics Co., Ltd. (72) Inventor ▲ Koji Takada 1-1, Kochi-cho, Takatsuki-shi, Osaka No. Matsushita Electronics Co., Ltd. (72) Inventor Takashi Kunimatsu 1-1, Satsukicho, Takatsuki-shi, Osaka Prefecture F-term (reference) 5F038 AV02 AV03 AV05 AV06 BG02 BG04 BG06 BH06 BH11 DF08 EZ20 5G065 AA01 DA06 DA07 EA06 HA04 JA01 KA02 KA05 LA01 LA02 MA01 MA03 MA09 MA10 NA01 NA09 5H730 AA14 AS01 AS04 AS23 BB43 BB57 CC01 DD04 EE02 EE07 EE59 FD01 FF19 FG05 VV01 VV03 ZZ05 ZZ11 ZZ12

Claims (7)

[Claims] A transformer, a switching element having an input terminal connected to a primary winding of the transformer, receiving a first DC voltage via the transformer, and a secondary winding of the transformer. An output voltage generation circuit that is connected and rectifies and smoothes an output voltage on the secondary side of the transformer to generate and output a second DC voltage from the first DC voltage; A control circuit that controls the operation of the second DC voltage; an output voltage detection circuit that detects and outputs a voltage value of the second DC voltage; and an output signal of the output voltage detection circuit is fed back to the control circuit in an insulated state as a feedback signal. A feedback circuit that receives an output of the primary winding on an anode, receives an output of the switching element on a cathode, and generates a power supply voltage for the control circuit; and a power supply capacitor for a control circuit. Output side and the control circuit A voltage regulator provided between the anode of the power supply capacitor for use and holding the power supply voltage at a predetermined value, wherein the control circuit generates and outputs a switching signal to be applied to the switching element; and A current detection circuit that detects a current flowing through the switching element and outputs it as an element current detection signal; an output load detection circuit that converts the feedback signal into a feedback voltage signal indicating a secondary-side load state and outputs the feedback voltage signal; An error amplifier that generates and outputs an error voltage signal composed of a difference between a voltage signal and a reference voltage, and compares the element current detection signal with the error voltage signal.
A comparator that outputs a compared comparison signal; a switching signal control circuit that controls the output of the switching signal based on the comparison signal; and a switching signal control circuit when the error voltage signal is smaller than a lower limit voltage value. Light load detection that stops outputting the switching signal to the switching element and starts outputting the switching signal to the switching signal control circuit when the error voltage signal is larger than an upper limit voltage value. And a switching power supply device. 2. The switching power supply according to claim 1, wherein the feedback circuit includes a photocoupler. 3. The switching power supply according to claim 1, wherein the light load detection circuit includes a detection voltage varying unit that can variably set a value of the lower limit voltage or the upper limit voltage. apparatus. 4. The switching element and the control circuit, comprising: an input terminal and an output terminal of the switching element; an input terminal on the anode side of the control circuit power supply capacitor in the control circuit; and the feedback in the output load detection circuit. The switching power supply device according to any one of claims 1 to 3, wherein the signal input terminal is formed integrally on one semiconductor substrate so as to be an external connection terminal. 5. A transformer, a switching element having an input terminal connected to a primary winding of the transformer, receiving a first DC voltage via the transformer, and a secondary winding of the transformer. An output voltage generation circuit that is connected and rectifies and smoothes an output voltage on the secondary side of the transformer to generate and output a second DC voltage from the first DC voltage; A control circuit that controls the operation of the second DC voltage; an output voltage detection circuit that detects and outputs a voltage value of the second DC voltage; and an output signal of the output voltage detection circuit is fed back to the control circuit in an insulated state as a feedback signal. A feedback circuit that receives an output of the primary winding on an anode, receives an output of the switching element on a cathode, and generates a power supply voltage for the control circuit; and a power supply capacitor for a control circuit. Output side and the control circuit A switching power supply semiconductor device that is provided between the power supply capacitor and an anode of the switching power supply capacitor, and controls a switching power supply device including a voltage regulator that holds the power supply voltage at a predetermined value. The control circuit includes the switching element and the control circuit, wherein the control circuit generates and outputs a switching signal to be applied to the switching element, and detects a current flowing through the switching element and outputs a current as an element current detection signal. A detection circuit, an output load detection circuit that converts the feedback signal into a feedback voltage signal indicating a secondary-side load state and outputs the error voltage signal, and generates an error voltage signal including a difference between the feedback voltage signal and a reference voltage. An error amplifier to output, comparing the element current detection signal and the error voltage signal,
A comparator that outputs a compared comparison signal; a switching signal control circuit that controls the output of the switching signal based on the comparison signal; and a switching signal control circuit when the error voltage signal is smaller than a lower limit voltage value. Light load detection that stops outputting the switching signal to the switching element and starts outputting the switching signal to the switching signal control circuit when the error voltage signal is larger than an upper limit voltage value. And a circuit for a switching power supply. 6. The switching power supply semiconductor according to claim 5, wherein the light load detection circuit includes a detection voltage varying unit that can variably set a value of the lower limit voltage or the upper limit voltage. apparatus. 7. The switching element and the control circuit, comprising: an input terminal and an output terminal of the switching element; an input terminal on the anode side of the control circuit power capacitor in the control circuit; and the feedback in the output load detection circuit. 7. The semiconductor device for a switching power supply according to claim 5, wherein a signal input terminal is integrated on one semiconductor substrate so as to be an external connection terminal.